From 59d5dcca1dcb7b83a8e461e109e2e8ddfbef6eeb Mon Sep 17 00:00:00 2001
From: Eric Curtin <ericcurtin17@gmail.com>
Date: Mon, 6 Jan 2020 11:23:29 +0000
Subject: [PATCH] heapsort implementation started at index 1

It is wasteful of the first element of the array, so start at 0.
Also std:swap was re-implemented just use swap available in C++ library
instead.
---
 sorting/heap_sort.cpp | 54 +++++++++++++++++++++----------------------
 1 file changed, 26 insertions(+), 28 deletions(-)

diff --git a/sorting/heap_sort.cpp b/sorting/heap_sort.cpp
index 572f5b5ce..9948bb821 100644
--- a/sorting/heap_sort.cpp
+++ b/sorting/heap_sort.cpp
@@ -1,54 +1,52 @@
+#include <algorithm>
 #include <iostream>
 
-void max_heapify(int *a, int i, int n) {
-    int j, temp;
-    temp = a[i];
-    j = 2 * i;
-    while (j <= n) {
-        if (j < n && a[j + 1] > a[j])
-            j = j + 1;
-        if (temp > a[j]) {
-            break;
-        } else if (temp <= a[j]) {
-            a[j / 2] = a[j];
-            j = 2 * j;
-        }
+void heapify(int *a, int i, int n) {
+    int largest = i;
+    const int l = 2 * i + 1;
+    const int r = 2 * i + 2;
+
+    if (l < n && a[l] > a[largest])
+        largest = l;
+
+    if (r < n && a[r] > a[largest])
+        largest = r;
+
+    if (largest != i) {
+        std::swap(a[i], a[largest]);
+        heapify(a, n, largest);
     }
-    a[j / 2] = temp;
-    return;
 }
 
 void heapsort(int *a, int n) {
-    int i, temp;
-    for (i = n; i >= 2; i--) {
-        temp = a[i];
-        a[i] = a[1];
-        a[1] = temp;
-        max_heapify(a, 1, i - 1);
+    for (int i = n - 1; i >= 0; --i) {
+        std::swap(a[0], a[i]);
+        heapify(a, 0, i);
     }
 }
 
 void build_maxheap(int *a, int n) {
-    int i;
-    for (i = n / 2; i >= 1; i--) {
-        max_heapify(a, i, n);
+    for (int i = n / 2 - 1; i >= 0; --i) {
+        heapify(a, i, n);
     }
 }
 
 int main() {
-    int n, i;
+    int n;
     std::cout << "Enter number of elements of array\n";
     std::cin >> n;
     int a[20];
-    for (i = 1; i <= n; i++) {
-        std::cout << "Enter Element " << (i) << std::endl;
+    for (int i = 0; i < n; ++i) {
+        std::cout << "Enter Element " << i << std::endl;
         std::cin >> a[i];
     }
+
     build_maxheap(a, n);
     heapsort(a, n);
     std::cout << "Sorted Output\n";
-    for (i = 1; i <= n; i++) {
+    for (int i = 0; i < n; ++i) {
         std::cout << a[i] << std::endl;
     }
+
     std::getchar();
 }